I think we all know that kayokyoku as a genre is a rather broad one, and basically encompasses anything sung that isn't enka, folk, rock, or explicitly some other genre it could be.
From my listening experience with kayo, I've been noticing that, for the most part, it's largest influences were from European popular music (which I have only limited exposure to myself), and American/British popular music. This was pretty evident in the 1960s and early 1970s, where the music sounded like stuff that the Western music world has by that time written off as "old people music".
I'm not insinuating that kayo was being derivative of Western styles, but they were readily adopted and had musical idioms and styles that were more towards Japanese tastes added to it. If you listen to, say, Jun Mayzumi, Naomi Chiaki, Chiyo Okumura, Kiyohiko Ozaki, or even Momoe Yamaguchi before singing songs by Ryudo Uzaki, the style of kayo was primarily Western in character, even if compared to contemporary Western music they may sound outdated.
The tone of kayo, I think, changed dramatically in the mid 1970s, and really broke out in 1975, when the ever-adapting musical chamelon Kyohei Tsutsumi started writing songs in his own voice, a voice that is wholly within Japan's scope of music, but can't be pinned down entirely or applied directly to Western music.
I don't know exactly how to qualify why I think 1975 is the year when this happened, but just by listening, it seems that a good number of new songs that came out then just "sounded" very different from what came before. The melodies were longer and complex, the arrangements blended electric and acoustic together, and most importantly, the music seemed to have updated significantly from the more crooner and cabaret kayo style that was predominant up until then.
I present four great examples from 1975 that encompass this change. "Romance" and "Momen no Handkerchief" are both Kyohei Tsutsumi songs (lyrics by Yuu Aku and Takashi Matsumoto respectively, two giants in songwriting themselves). Before then, his music, though showing his voice in his own was still strongly rooted in Western pop tradition, such as "Blue Light Yokohama" or "Mebae". Tsutsumi showed some experimental signs of forging a more unique sound in 1974, particularly in "Nigai Namida", his song written for the Three Degrees. Since then, his style has diversified and flourished, and his music writing can adapt to many idioms, though I believe these two songs are by far some the best examples of the beginning of his mature career in songwriting.
"Toki no Sugiyuku Mama ni" by Kenji Sawada was a big departure in balladry. Here we have a song that still has the sort of intimacy and depth of a classic ballad, but with very little direct referencing to the older ballad styles that seem crooner-esque (see Goro Noguchi's "Shitetsuensen" from the same year), and in fact sounds distinctly Japanese in character. It's not necessarily the first song to do this (Kei Ogura's "Cyclamen no Kaori" made popular by Akira Fuse did this first, but that may be considered New Music), but it was the first notable in the kayo context.
Lastly, as an example of a transition between the old and new, we have "Yureteru Watashi" by Junko Sakurada. This song, composed by Kouichi Morita (made songs for Mari Amachi and Candies), is definitely a more pop-sounding kayo number, but unlike some of his earlier work, it too seems to lack the strong connections to Western styles and appears to be of its own brand. Morita wouldn't really strike heavily into modern kayo until 1976 with "Haru Ichiban" by Candies, and Sakurada's compatriot Momoe Yamaguchi also would shift her song style that same year when singing Ryudo Uzaki's compositions, most notably in "Yokosuka Story".
I encourage any discussion on my theory that the style shift in kayokyoku in 1975 is something detectable, or perhaps if there are other examples that would place that time somewhere else. It's pretty subjective, but I think there's something to it when everything is listened in a chronological fashion.
Hiromi Iwasaki - Romance [岩崎宏美 - ロマンス] (Yoru no Hit Studio)
Kenji Sawada - Toki no Sugiyuku Mama ni [沢田研二 - 時の過ぎゆくままに] (Yoru no Hit Studio)
Junko Sakurada - Yureteru Watashi [桜田淳子 - ゆれてる私] (Yoru no Hit Studio)
Hiromi Ohta - Momen no Handkerchief [太田裕美 - 木綿のハンカチーフ]
- 1975, when Kayokyoku gains its own identity